A thermal head comprises a plurality of resistors heated by electrical dissipation, and performs a printing operation by applying heat energy generated by the heating resistors to a print medium. Conventionally, a head substrate and a printed circuit board are separately provided, and an aluminum heat-dissipation plate is adhered to the lower surface of the head substrate and the lower surface of the printed circuit board. The head substrate has the plurality of heating resistors, a common electrode connected in common to one end of the plurality of heating resistors, and a plurality of individual electrodes connected to the other end of the respective heating resistors. The printed circuit board has a plurality of drive elements which control the heating of the heating resistors via the individual electrodes at the head substrate. In order to achieve high-speed printing, it is necessary to increase the thermal response of the heating resistors by discharging excess heat via the aluminum heat-dissipation plate. Structures of thermal heads comprising heat-dissipation plates are disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 1-190467 and 11-28829 and Japanese Unexamined Utility Model Registration Application Publication No. 1-131538 (microfilm).
In recent years, a reduction in the size of a thermal head has caused a reduction in the size of a head substrate. However, when the size of the head substrate is reduced, the heating effect of heating resistors does not change, whereas the contact area of the head substrate and a heat-dissipation plate is reduced. Therefore, the quantity of heat transmitted to the heat-dissipation plate from the head substrate via an adhesive layer when printing is carried out is relatively increased. This excess heat expands the length of the heat-dissipation plate, as a result of which the adhesive layer may become peeled depending upon the conditions. The amount of increase in the length is particularly large at the ends of the heat-dissipation plate. As shown in FIG. 7, this results in a reduction in shear strength of the adhesive, and the adhesive layer tends to become peeled. In FIG. 7, the measuring points are distributed over the length of the head substrate. The tendency to peel becomes more noticeable as the heating temperature increases. Air tends to enter peeled areas of the adhesive layer, and such areas act as heat accumulators, causing the temperature of the heating resistors to increase. Therefore, the print density of the heating resistors disposed at the ends increases, thereby resulting in inconsistencies in the print density as a whole.
In addition, when the size of the head substrate is reduced, the head substrate becomes less resistant to bending. For example, when an attempt is made to adhere a printed circuit board and the head substrate to the heat-dissipation plate after wire bonding and sealing individual electrodes and drive elements, a large warping occurs in a plane direction of the head substrate by thermal contraction of a sealing resin, thereby deteriorating print quality such as by causing print fading. Conventionally, the area of the head substrate is at least 6 mm2. Therefore, even if the sealing resin undergoes thermal contraction, the head substrate does not tend to bend. Consequently, even if warping occurs in a plane of the head substrate, the warping is restricted within a range not affecting the print quality.
Warping in a plane direction of the head substrate may be restricted by previously adhering the head substrate and the printed circuit board to the heat-dissipation plate, and, while an adhesive prevents the head substrate from bending, by wire-bonding and sealing the individual electrodes and drive elements. Conventionally, the individual electrodes, formed of aluminum (Al) to reduce costs, and the drive elements are connected with a gold (Au) joining wire. The Al—Au wire bonding is carried out in an atmosphere of approximately 150° C. in order to form an Al—Au alloy layer by breaking surface oxidation layers of the individual electrodes by ultrasonic vibration. Therefore, the adhesive layer may become peeled as a result of the expansion of the heat-dissipation plate in this high-temperature atmosphere. As described above, the peeled areas of the adhesive layer act as heat-accumulation layers due to air entering the peeled areas. Therefore, there are inconsistencies in the print density.